Closeup filters are unique...they make the camera near sighted.
What they do is shorten the cameras focus range...for instance, with the focus set at infinity with your +4 diopter the camera can only focus a maximum of 1/4th of a metre or 25cm (9.8")...having two +4 diopters on the camera will mean at infinity the furthest distance away the camera will focus to is 1/8th of a metre or 12.5cm (4.92").
The real trick with using diopters is to use the camera's zoom...for instance, here's a picture I took using an Olympus C-700 with +7 diopter (+4, +2, +1 combined), I was 13cm/5" away but I had the zoom at 300mm:
http://img4.photobucket.com/albums/0...up/uspenny.jpg
Try zooming in.